Precision measurements of rare pion decays in the PIENUX experiment.

Precision measurements of rare decays of charged pions enable important standard model tests. The branching ratio of π⁺ → e⁺ ν to π⁺ → μ⁺ ν decay provides the best test of electron–muon universality. This ratio, due to helicity suppression via the Lorentz structure of the weak current, is also uniquely sensitive to exotic pseudoscalar and scalar currents. Pion beta decay π⁺ → π⁰ e⁺ ν, a weak process analogous to superallowed Fermi transitions in nuclear beta decay, offers the smallest theoretical uncertainties for the extraction of the V_ud matrix element of the CKM matrix.

Herein we discuss the plans by the PIENUX collaboration for a ten-fold improvement in the branching ratio determinations for the rare π⁺ → e⁺ ν and π⁺ → π⁰ e⁺ ν  decays. Using stopped pions, PIENUX will perform measurements of the relative yields of 70 MeV monoenergetic positrons from π⁺ → e⁺ ν decay, ~70 MeV coincident photons from π⁺ → π⁰ e⁺ ν decay, and 0-53 MeV Michel positrons from π⁺ → μ⁺ → e⁺ decays.  The measurements demand high statistics and extensive suppression of systematic effects. Emergent technologies for both electromagnetic calorimetry and 4-dimensional, high-granularity tracking of pions, muons and positrons will be used.